Abstract [en]

The exchange of momentum between the oceans and atmosphere is important for many atmospheric and oceanic processes and is mainly governed by the roughness of sea surface. The roughness can be expressed by a roughness length z0. A roughness length model, based on the concept that z0 is determined by stochastic wave breaking, is presented. The model performance is evaluated using measurements from the Östergarnsholm site, in the Baltic Sea, and pertinent information from other recent investigations. The wave field and the roughness length variations are investigated during various sea state conditions dominated by wind-driven waves. It is found that several parameters, describing the characteristics of the wave field, are dependent on the amount of energy that long waves have relative to the energy of short, wind-driven waves of the sea spectrum (called the swell ratio). The impact of swell ratio on z0 can explain the discrepancies found in various results among relevant investigations. The roughness length model can well reproduce the observed roughness length.